Recent developments in the field of radiotherapy have focussed on integrating an imaging system with the therapeutic system. The goal is to provide real-time feedback on the location of an anatomical feature within the patient (e.g. a tumour) such that a therapeutic radiation beam can be more accurately controlled to target that feature, One suggested approach is to combine a linear accelerator-based therapeutic system with a magnetic resonance imaging (MRI) system within a single apparatus, known as an MRI-Linac. Such apparatus is described in a number of earlier applications by the present Applicant, including U.S. patent application Ser. No. 12/704,944 (Publication No. 2011/0201918) and PCT Publication No. 2011/127947. In the system described in each of these earlier applications, the magnetic coils of the MRI system are split, leaving a gap through which a therapeutic radiation beam can be delivered to the patient. In this way, the patient can be imaged and treated substantially simultaneously while lying in the same position.
If the MRI system is to provide reliable information to the therapeutic system, it is important that the two systems are accurately calibrated; that is, the coordinate system of the MRI system must be registered to that of the treatment beam so that measurements in the MRI system can be translated into instructions in the therapy system.
Phantoms are known devices which are scanned or imaged to evaluate and tune the performance of various medical imaging devices. A paper by Rhode et al (“Registration and Tracking to Integrate X-Ray and MR Images in an XMR Facility”, IEEE Transactions on Medical Imaging, Vol. 22, pages 1369-1378) describes a method of registering x-ray and MR images in which a phantom is first imaged in an x-ray system before being translated a distance and imaged by an MRI system. The distance between the two systems is measured to enable the two coordinate systems to be co-registered. When in the x-ray system, ball bearings are used as markers within the phantom; when in the MRI system, the ball bearings are replaced by MR imaging markers to avoid problems arising from interactions with the intense magnetic field. This system has several drawbacks. The replacement of the ball bearings with MR imaging markers introduces a potential source of error if the two markers are not exactly co-located within the phantom. In addition, the translation of the phantom between the two devices introduces a further source of error.